This invention relates to air duct silencing apparatus for connection to the intake of an air supply fan unit for a building or other large structure.
Air duct silencers of various types are well known in the air handling industry. A number of these silencers are designed for attachment either to the inlet or the outlet of an air supply fan which is capable of creating substantial noise during its operation. One purpose of these silencers is to prevent noise from the fan from passing through the air duct system of the building and thereby disturbing people who may be dwelling or working in the building. It is important that these air duct silencers not only be capable of reducing the level of noise from the fan substantially but also that they accomplish this objective efficiently so that the necessary air supply can still be delivered by the fan unit to various areas in the building. It will be appreciated that if the air duct silencers are not very efficient, it may be necessary to increase the size of the fan unit in order to deliver the quantity of fresh and/or return air required by the building. As larger fans require more electrical energy in order to operate, it is generally desirable not to increase the size of the fan unit any more than necessary.
In the case of an inlet silencer for a fan unit, it is desirable to have the velocity of the airflow entering the inlet of the silencer relatively low as this results in a velocity variant between heat exchanger coils, which are often located a short distance upstream of the inlet silencer, and the unit of the silencer being small. As a result, there will be achieved relative uniformity of airflow velocity across the face of the heat exchanger coils despite the short distance between the adjacent face of these coils and the inlet of the silencer. Other desirable objectives of an air handling silencer are the existence of a low pressure drop across the silencer and the lack of upstream wakes or flow distortion created by splitters or similar air attenuating devices in the airflow passage.
Other desirable objectives that are often sought in the design and construction of air inlet or air outlet silencers include the provision of a compact silencer since often the amount of available space for these silencers is quite limited and the use of a silencer design which requires fewer materials and less labor to build, thus reducing overall costs.
One known type of inlet silencer for a fan unit is that manufactured and sold by M and I Heat Transfer Products Ltd. of Mississauga, Ontario, Canada, and disclosed in its Compac Space Fan System brochure. In this inlet silencer, which is generally designed to sit on the floor below the fan unit, there is an annular air inlet that can be open on four vertical sides of the air inlet apparatus and an annular air outlet in the top of the housing that forms the air inlet silencer. Located in the centre of the housing is a substantially conical airflow defining member with a wide circular base and a narrow circular top adjacent the inlet of the fan. There are curved, perforated interior walls that form an outer section of the air inlet silencer and that surround the inner airflow defining member. The inner airflow defining member is filled with sound attenuating material and similar material is also located behind the interior walls of the outer section.
German Offenlegungsschrift 3401 210 A, describes a silencer that can be mounted either at the entrance to or the exit from an axial fan unit. This silencer also has an exterior housing that contains sound-absorbent material and an annular duct extends from one end of this housing to the opposite end. This silencer has a central airflow defining member that has first and second longitudinal sections with the first longitudinal section having a generally conical shape and the second, shorter section being cylindrical. The outer end of the airflow defining member is quite wide with the outer end surface being planar and extending transversely to a longitudinal central axis of the airflow defining member. In this air duct silencer, the cross-sectional flow area of the annular duct is indicated to be constant over the entire length of the silencer and may be equal to or slightly greater than that of the annular passage forming the fan inlet. The airflow defining member in this silencer is filled with sound-absorbent material.
Recent U.S. Pat. No. 5,426,268 issued Jun. 20, 1995 to M Yazici et al describes both an inlet silencer and an outlet silencer for an axial fan unit. The duct inlet apparatus described in this patent, which apparatus can also be used for an outlet silencer, includes an exterior housing with two principal air inlets located on opposite vertical sides of the housing. The silencer has a single annular air outlet located at one end of the housing and this outlet is connected to the inlets by main airflow passageways defined by interior walls that are preferably perforated. Sound insulating material can be placed behind the interior walls. Preferably each inlet is divided into four generally rectangular segments of similar size. It will be appreciated that this inlet silencer is reasonably complex and can be time consuming to construct.
It is an object of the present invention to provide an air duct silencing apparatus wherein the velocity of airflow entering the inlet of the apparatus can be relatively low and wherein the pressure drop across the length of the apparatus is reasonably low.
It is a further object of the present invention to provide an air duct silencing apparatus that is reasonably compact and that can be constructed at a reasonable cost while at the same time providing good sound attenuating characteristics.
It is a further object of the present invention to provide substantially uniform inlet flow to a fan""s blades so that the fan is able to achieve high performance in terms of both aerodynamics and acoustics.
According to one aspect of the invention, an air duct silencing apparatus for use as an inlet silencing duct to be connected to an air supply unit for a building or other large structure includes an exterior housing having exterior walls forming outer surfaces of the housing, an air inlet formed on one end of the housing and a circular air outlet formed in an opposite end of the housing and adapted for connection to the fan unit for airflow to the fan unit. The outlet defines a primary central axis extending through the center of the outlet and perpendicular to a plane in which the outlet lies. Interior walls are arranged in the housing and are connected to the exterior walls. These interior walls define an annular airflow passageway extending from the air inlet to the air outlet and a substantial portion of these walls are made of perforated metal. There is also a central airflow defining member having a second central axis extending from an outer end thereof to an inner end adjacent the air outlet. This second central axis is substantially coaxial with the primary central axis. The airflow defining member is substantially circular in transverse cross-section along its length and has a relatively wide bulbiform end section at its outer end. The airflow defining member also has a relatively narrow end section at its inner end. Sound absorbing material is provided in the housing and is covered by the interior walls and is also located in the airflow defining member. The airflow passageway tapers inwardly in the direction of the air outlet and the cross-sectional flow area of this passageway varies along at least a substantial portion of its length so that airflow speed at the air inlet is low relative to airflow speed at the air outlet when the air duct silencing apparatus is connected to the fan unit and the fan unit is operating.
In a preferred embodiment, the central airflow defining member projects outwardly from the air inlet. Also, the airflow passageway is tapered and the airflow defining member is curved as seen in an axial plane extending through the second central axis so that the airflow speed increases smoothly to maximum velocity at the air outlet when the fan unit is operating.
According to another aspect of the invention, there is provided an air duct silencing apparatus adapted to be connected to an air supply fan unit, this apparatus having a housing with two opposite end walls, an air inlet formed in one of these end walls, and a smaller air outlet formed in the other end wall where the fan unit is located during use of the silencing apparatus. Interior walls are arranged in the housing and define an annular, tapered airflow passageway extending from the air inlet to the air outlet. The apparatus is provided with an improved airflow defining member located centrally in the airflow passageway and extending the length of the airflow passageway. This member has a substantially circular transverse cross-section and has first and second longitudinal sections, with the first longitudinal section being relatively wide with side walls that are curved along the length of the section in axial planes extending through a central longitudinal axis of the member. The second longitudinal section is relatively narrow and cylindrical and located adjacent the air outlet. The cross-sectional flow area of the annular airflow passageway varies along its length so that the airflow speed at the air inlet is low relative to airflow speed at the air outlet during use of the silencing apparatus.
Preferably, the first longitudinal section is substantially bulb-shaped and the airflow defining member is filled with sound insulating material.
Further features and advantages of the air duct silencing apparatus of the invention will become apparent from the following detail description taken in conjunction with the accompanying drawings.